test: add unit tests for src/cost.ts

[doitrous]

Adds test/cost.test.ts covering the four exports of src/cost.ts:

• JEV_USD_PER_MTOK: matches the published rate
• newMeter: starts at zero
• addJevCost: correct token→USD math, accumulation, edge cases (NaN, Infinity, negative, zero, non-numbers, numeric strings), undefined meter safety
• addUsd: exact addition, accumulation, same edge cases

12 tests, all pass under bun test.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Tests
  • Added coverage for cost calculations, including token-to-USD conversion and accumulated totals.
  • Verified handling of numeric strings and invalid inputs.
  • Confirmed meter initialization and exact USD additions.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 7a453d45-db57-4ae9-9186-368f509908b8

📥 Commits

Reviewing files that changed from the base of the PR and between 5c241af and abea66b.

📒 Files selected for processing (1)

• test/cost.test.ts

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📝 Walkthrough

Walkthrough

The pull request adds test/cost.test.ts. The tests cover the exported JEV rate, meter initialization, JEV cost conversion, USD additions, accumulation, numeric strings, and invalid inputs.

Changes

Cost module tests

Layer / File(s)	Summary
Cost meter behavior tests test/cost.test.ts	Tests verify JEV_USD_PER_MTOK, newMeter(), addJevCost, and addUsd. The tests cover conversion, accumulation, numeric strings, invalid values, and an undefined meter.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Other

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding unit tests for src/cost.ts.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

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